Liquid mixing is an indispensable process in chemical, pharmaceutical, food and beverage industries, and the accuracy of the mixing ratio and the uniformity of mixing directly affect product quality and production efficiency. However, most liquid mixing operations involve flammable, explosive, toxic or corrosive media, and the harsh on-site environment makes manual operation unsuitable, while manual and semi-automatic control suffers from low proportioning accuracy and insufficient automation. To address this problem, this paper takes the proportional mixing of three kinds of vegetable oils as the research object and designs and simulates a liquid mixing control system based on the Siemens S7-1200 PLC. The system adopts the stirring-motor mixing method, realizes dual detection of liquid level and weight with a floating-ball level sensor and a pressure sensor, compiles the control program with the sequential control design method and step ladder instructions, and completes configuration simulation with TIA Portal V16 and WinCC. Simulation results show that with a set output of 80 g and a mixing ratio of 1:2:3 for materials A, B and C (10 g, 20 g and 30 g), the single-cycle actual output is 60 g; the system automatically completes feeding, weighing, stirring and discharging and repeats feeding cycles until the cumulative output reaches the set value. The stirring motor runs alternately in forward rotation for 5 s and reverse rotation for 5 s, giving uniform mixing and stable operation. The system is simple in structure, low in cost and highly automated, and can serve as a reference for the automation of small and medium-sized liquid mixing processes.